An ankle joint may become severely damaged and painful due to arthritis from prior ankle surgery, bone fracture, infection, osteoarthritis, posttraumatic osteoarthritis or rheumatoid arthritis, for example. Options for treating the injured ankle have included anti-inflammatory and pain medications, braces, physical therapy, amputation, joint arthrodesis, and total ankle replacement.
In the past the main-stay of ankle arthrosis has been joint arthrodesis, due to the poor prosthetic survival rate of total ankle replacements. This is primarily due to the clinical results of early ankle designs. Therefore, arthrodesis has been the only choice for many surgeons and patients. Arthrodesis improves stability and reduces pain, but also severely inhibits normal function of the ankle joint. Although some patients have very good results from ankle fusion, surrounding joints above and below the fusion may become arthritic and painful because the lack of ambulation places additional stress on these joints.
There have been numerous ankle joint replacement prostheses developed over the last 30 years. The Agility ankle is an example of an early implant design. It is comprised of two components—one part is cemented to the tibia and the other part is cemented to the talus. An issue with the design of the early ankle prostheses is that although they allow for some dorsiflexion/plantarflexion motion, the articulation surfaces restrict varus/valgus and rotation motions. Another problem surrounding early implant designs is their reliance on the surrounding soft tissues of the ankle to stabilize the implant. As a result, they are not well suited for implantation in individuals with compromised soft tissues.
Another example is the Salto Talaris ankle device, which is a fixed-bearing ankle prosthesis. This two-component ankle system utilizes a conical talar component with two different radii of curvature and a curved groove in the sagittal plane. The medial radius is smaller than the lateral to allow equal tensioning of the collateral ligaments. The tibial component is designed for a fixed insertion of a polyethylene bearing piece that is replaceable. Some issues with the fixed-bearing prostheses include high wear rate of the articulation surfaces, ambulatory constraint, and loosening of the implant.
Another ankle replacement device is the Scandanavian Total Ankle Replacement (STAR). In this device the tibial component is designed for less bone resection and has two parallel bars for insertion into the subchondral bone. The talar component is meant to mimic the talar dome and has a central ridge for stabilization of a polyethylene piece. The STAR prosthesis inhibits inversion/eversion coupling with plantarflexion/dorsiflexion motion. This leads to straining and potential damage to the deltoid ligaments on the medial side of the ankle. Another issue with this device is edge loading, which puts a great amount of stress on the ridge of the implant and results in the implant retracting from the talus.
More modern designs have attempted to increase the range of motion while maintaining the integrity of the surrounding soft tissues of the ankle. For example, U.S. Pat. No. 7,625,409 discloses a prosthesis designed to allow full range of motion while minimizing edge loading and subsidence. However, this prosthesis fails to address the need for an implant for use in patients with compromised soft tissues in the ankle.